Conventional oil recovery involves drilling a well and pumping a mixture of oil and water from the well. Oil is separated from the water, and the water is usually injected into a sub-surface formation. Conventional recovery works well for low viscosity oil. However, conventional oil recovery processes do not work well for higher viscosity, or heavy, oil.
Enhanced Oil Recovery (EOR) processes employ thermal methods to improve the recovery of heavy oils from sub-surface reservoirs. The injection of steam into heavy oil bearing formations is a widely practiced EOR method. Typically, several tons of steam are required for each ton of oil recovered. Steam heats the oil in the reservoir, which reduces the viscosity of the oil and allows the oil to flow to a collection well. The steam condenses and mixes with the oil, the condensed steam being called produced water. The mixture of oil and produced water that flows to the collection well is pumped to the surface. Oil is separated from the produced water by conventional processes employed in conventional oil recovery operations.
For economic and environmental reasons it is desirable to recycle the produced water used in steam injection EOR. This is accomplished by treating the produced water, producing a feedwater, and directing the treated feedwater to a steam generator or boiler. The complete water cycle includes the steps of:                injecting the steam into an oil bearing formation,        condensing the steam to heat the oil whereupon the condensed steam mixes with the oil to become produced water,        collecting the oil and produced water in a well,        pumping the mixture of oil and produced water to the surface,        separating the oil from the produced water,        treating the produced water so that it becomes the steam generator or boiler feedwater, and        converting the feedwater into steam that has a quality of approximately 70% to 80% for injecting into the oil bearing formation.        
Treating the produced water to form a relatively pure distillate as feedwater in steam generation is challenging. It is known to chemically treat the produced water and subject the produced water to an evaporation process to form the distillate for steam generation feedwater. In particular, it is known to use a single evaporator and mechanical vapor compressor to produce the distillate. However, there are drawbacks and disadvantages to a single evaporator and mechanical vapor compression process. These systems tend to be expensive to operate and costly to maintain.
Therefore, there is a need in heavy oil recovery processes for a more cost-effective evaporation system for treating produced water and producing a relatively pure feedwater stream for a steam generation system.